A common type of digestion tank comprises a mixer assembly for agitating the excrement, or mixture of fluid and sludge, present in the chamber. Such applications comprise tanks having a depth from about 10 meters to about 40 meters, however a common depth is about 15-25 meters. Furthermore such tanks have a volume of several thousands of cubic meters, in which excrements, feaces, sludge, waste water, or the like is stored and digested in order to extract methane gas and carbon dioxide, for example. The liquid/sludge is kept in movement by agitation in order to not having the sludge to sediment and in order to not causing a dense cake at the surface of the liquid/sludge, and thereto in order to obtain an as homogenous mixture as possible which entail that most possible quantities of various gases may be extracted. It shall be pointed out that other materials than excrement suspensions, such as fluid mixtures, industrial suspensions, solid material in the form of powder or granulate, may be stored in similar conditions in which agitation takes place. Common for the applications for which the present invention is intended is that the maintenance staff cannot, are not allowed to or does not want to perform work in the chamber during long periods of time, for instance one or several years.
Mixer assemblies suitable for use in a digestion tank or the like comprises a motor and a gear box that lowers the rotational speed of the output shaft of the mixer assembly to about 15-30 revolutions per minute, even though other speeds may be used. The output shaft rotatably suspended from the drive unit of the mixer assembly and usually hang down freely straight down into the chamber, whereupon the lower end of the output shaft usually is located at a distance of 6-12 meters from the floor of the chamber, and whereupon a propeller unit is located at the lower end. Such propeller units may have a diameter of 1-6 meters, and the output shaft may have a length of for instance about 10-30 meters and a diameter of for instance 10-30 centimeters.
If an unbalance appear in the mixer assembly, i.e. if the output shaft starts to perform a nutatinq movement or starts to deflect/oscillate during rotation or wriggle, and the mixer assembly is not immediately turned off, the mixer assembly as well as the chamber runs the risk of getting damaged. Unbalance may for instance arise due to the fact that a blade has come loose from the propeller unit, that solid material has become stock to the blades of the propeller unit, that a settlement/inclination of the tank takes place which makes the drive unit to incline in relation to a plumb line, etc.
A known way of preventing that a mixer assembly and/or the chamber will become damaged in the case such an unbalance arise is to lengthen the output shaft such that it extends towards and terminates at a distance from the floor, a steel ring having a for instance twice as big diameter than the diameter of the output shaft being arranged near to the lower end of the output shaft. If unbalance arises and the output shaft starts to wriggle, its wriggling/deflection will be limited by the steel ring. However, the steel ring or its attachment to the chamber will already after a, in this connection, short time be damaged if the mixer assembly is not turned off. This known solution also requires an unnecessarily long and expensive output shaft.
Unbalance may also arise if the output shaft gets into natural frequency due to the critical number of revolutions, this is usually prevented by choosing the length of the output shaft in such a way that this phenomena does not arise. However, this constructional limitation may lead to the fact that the optimal length of the specific application cannot be used due to the inherent natural frequency of the output shaft.